The Cold War started because Russia had traditionally been suspicious fo the West. This coincided with the Soviets’ interests in Europe. Russia had long looked to the West as a source of danger. This went back as far as Napoleon’s invasion in the 1810s. It was heightened by the fact that the West was so much more advanced than Russia. Because of this, Russia felt that it had an interest in maintaining a buffer zone in Eastern Europe that would insulate it from attacks from the West.
At the same time, the US had a tradition that was in conflict with this. The US had a tradition of believing that other countries (at least those with white populations) should govern themselves democratically. The US did not, therefore, like the idea of satellite states that were being forced by the Soviets into communism. This made the US suspicious of the Soviets. It made them fear that the Soviets were trying to dominate Europe. This caused the US to believe that it was in their interests to prevent Soviet expansion.
The conflict between these two sets of traditions and interests led to the Cold War.